1. Field of Invention
This invention relates generally to dc-link capacitors for inverter power modules, and more particularly to capacitor packaging features for electrical and thermal performance.
2. Background Art
Traction inverters for electric and hybrid electric vehicles typically include one or more high performance inverter circuits coupled to a power source via a passive electric device, such as a capacitor or capacitor bank that can store and release energy. The capacitor can smooth dc-link voltage, absorb ripple current, decouple power source inductance effects, and reduce the leakage inductance of the inverter power stage. To minimize parasitic or leakage inductance and optimize performance, a linking capacitor is typically positioned as close as possible to the semiconductor power switches of an inverter, with the size and design of the capacitor often driven by the size and power characteristics of the inverter to which it is linked. Inverter operation can generate heat that can be partially transferred to the link capacitor. Unfortunately, capacitor package thermal conductivity is generally rather poor, typically less than 1 W/m·K. Thus, even a small amount of heat transferred to the capacitor can significantly raise its temperature. Power module internal temperatures can reach 150° C., but most linking capacitors can only tolerate temperatures at or below 105° C. Consequently, capacitor temperature typically becomes an operational design constraint. As a result, inverter link capacitors are typically exaggerated in size to withstand the thermal challenges imposed by the inverter, and, as a consequence, increase the cost, size and weight of the inverter system.
FIG. 1A shows a traditional prior art inverter dc-link capacitor package 102 electrically coupled to an inverter power module 104. The package 102 is characterized by a length L, a height H and a width W. The traditional capacitor package 102 includes a resin layer 106 in which capacitor elements 108 are disposed, and a case 110 that can provide support for the package 102. A bus bar 112 has an internal portion 114 extending above and coupled to the elements 108. The bus bar 112 also includes an external portion 116 that extends externally to connect with the inverter power module 104. Heat transferred to the bus bar external portion 116 can, to a limited extent, radiate to the external environment; however, much of the heat is conducted to the internal portion 114. Due to the poor thermal characteristics of the package 102, the transferred heat can substantially raise the temperature of the capacitor elements 108. Unfortunately, high temperatures can cause a capacitor to breakdown at a lower voltage level. High temperatures can also adversely affect a capacitor's electrical performance and shorten its lifetime.
FIG. 1B shows an example equivalent circuit 120 for the traditional dc link capacitor package 102 that includes a series of thermal resistances R1-R5 between an IGBT terminal and a case/coolant. Based on the circuit 120, EQ. (1) below provides an expression for the maximum temperature of the traditional dc-link capacitor of the package 102:Tmax(Ctrad)=Tcoolant+(R3+R4+R5)*(R1+R2+R3+R4+R5)*(TPM−Tcoolant)  (EQ. 1)Where:
Tcoolant=coolant temperature
TPM=power module temperature
R1=thermal resistance of bus bar external portion 116 from IGBT terminal to dc-link capacitor internal bus bar portion 114;
R2=thermal resistance of resin from capacitor internal bus bar 114 to capacitor element 108;
R3=6.6 C/W, thermal resistance of capacitor element 108, from top to bottom;
R4=thermal resistance of epoxy 106 from capacitor element 108 bottom to case 110
R5=thermal resistance of capacitor case 110 to coolant if direct cooling, or to air if convection cooling
Example values for the variables listed above are provided below:
L=210 mm
W=160 MM
H=33 mm,
R1=0.54 C/W
R2=0.1 C/W
R3=6.6 C/W
R4=0.1 C/W
R5=0.4 C/W
Substituting the above values into EQ. 1 results in the simplified EQ. 2 below:Tmax(Ctrad)=Tcoolant+0.92*(TPM−Tcoolant)  (EQ. 2)
As shown by EQ. 2, the maximum temperature experienced by the example traditional capacitor can be over 90% of the temperature differential between the power module and the case, indicating that much of the heat produced at the power module can be transferred to the traditional linking capacitor. From EQS. (1) and (2) it can be seen that the traditional capacitor package offers a capacitor little protection against temperature increases caused by power module operation. The use of an inverter cooling plate with circulating coolant can reduce the temperature at the traditional capacitor. Nevertheless, because the thermal conductivity of the traditional capacitor package is fairly low, typically less than 1 W/m·K, it is difficult for the capacitor to radiate or dissipate heat, whether the heat is generated within the package by ripple currents, or transferred from an external power module. Even a few watts of heat can make a capacitor temperature rise dramatically. Experimental results indicate that in some circumstances a capacitor temperature can increase by 50° C. or more. Unfortunately, such temperature increases can have a significantly detrimental effect on capacitor longevity; every 10° C. temperature increase can be expected to shorten the capacitor lifetime by around 50%. In addition, some systems couple a plurality of inverters to a power source by the same linking capacitor, exposing the capacitor to heat generated by multiple inverter sources. As a result, to assure that a capacitor can tolerate high temperatures, traditional inverter designs often include capacitors sized much larger than electrical requirements alone would demand. Unfortunately, oversized capacitors increase the size and cost of an inverter power module.